There are conventionally known techniques for applying forward osmosis phenomenon to wastewater treatment, seawater desalination, osmotically-induced power generation, etc. For example, there is a known method of using a forward osmosis membrane for seawater desalination to dilute concentrated seawater that has been concentrated in a seawater desalination process (see, for example, Patent Literature 1). There is also a known power generation system, in which dilution water such as lower concentration seawater or freshwater is caused to penetrate, through a semi-permeable membrane, concentrated seawater that has been produced simultaneously with seawater desalination using a reverse osmosis membrane in a seawater desalination device, the flow rate of the concentrated seawater is increased by the positive osmotic pressure energy of the dilution water, and thereby power is generated using the increased flow rate of the concentrated seawater (see, for example, Patent Literature 2). There is also a known spiral-wound forward osmosis membrane element used for these systems using the forward osmosis phenomenon (see, for example, Patent Literature 3).
In a spiral-wound forward osmosis membrane element, a liquid moves from a lower concentration (lower osmotic pressure) solution to a higher concentration (higher osmotic pressure) solution through the membrane. In order to suppress formation of a concentration polarization layer near the membrane, it is necessary to cause the higher concentration (higher osmotic pressure) solution and the lower concentration (lower osmotic pressure) solution to flow over both surfaces of the membrane.
For example, in order to form a liquid flow on one of the surfaces of the membrane, the spiral-wound forward osmosis membrane element of Patent Literature 3 is configured to form a liquid flow from a central tube back to the central tube through a curved flow path in an envelope-like membrane leaf. In this membrane leaf, two curved flow paths are provided in parallel in the axial direction of the central tube.